The production of porcelain for electrical insulation — II

Abstract

The raw materials should be carefully tested before using. China and ball clays should be tested for fired color, porosity at the regular burning temperature, fineness of grain, etc. Ball clays should be tested for raw physical strength. The grain size of quartz is of particular importance and the fine grain sizes should be determined by elutriation or water separation. Feldspar should fuse to a glass with the fusion of pyrometric cone No. 8, 1280 deg. cent. (2336 deg. fahr.), and its color, degree of glassiness, amount of deformation, etc. noted. The composition and formation of porcelain in firing are briefly described. The limits of composition are wide but the quality of the final product will vary with the composition. Special porcelains for use as spark plug cores are made by eliminating feldspar and quartz and substituting synthetic calcines. During firing the mechanical water is first expelled. Chemically combined water is driven off at 500 deg. cent. (932 deg. fahr.). Alpha quartz assumes the beta form at 575 deg. cent. (1067 deg. fahr.) with a similar expansion in volume. Shrinkage and condensation of the volume of the clay substance takes place at 900 deg. cent. (1652 deg. fahr.). Continued firing contracts the porcelain and decreases the porosity, the feldspar finally melting and gradually taking the more refractory clay and quartz into solution or assisting in converting it into other materials, particularly the clay which breaks up into sillimanite and free silica. The solution of the quartz depends upon grain size and heat treatment. On account of volume change of the quartz grains, it is evident that the fired porcelain, in which the quartz grains are in intimate contact with the glassy groundmass will be placed in a condition of stress after cooling down as the quartz contracts more rapidly than the rest of the porcelain. Naturally the greatest strain occurs around the largest quartz grains and clearly demonstrates the necessity for fine grinding. The properties and testing of porcelain are described. Ordinary porcelain has a tensile strength of from 3000 to 6000 pounds per square inch and a coefficient of lineal thermal expansion of from 4 to 9 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−6</sup> per degrees centigrade, while special porcelains have a strength as high as 12,000 pounds and an expansion as low as 2.7 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−6</sup> .